Since the 1930s a number of methods have been disclosed for producing aliphatic alcohols, alicyclic alcohols or aromatic alcohols by hydrogenating carboxylic acids or esters of carboxylic acids. In those methods, copper catalysts are mainly proposed for use in hydrogenation of esters of carboxylic acids, particularly fatty acid esters, and copper-chromium catalysts are commonly used for industrial purposes.
The conditions employed to activate these catalysts by reduction are determined depending on the form of catalysts and usage, reduction method and other factors. For example, when the fluidized bed reaction system is employed, a catalyst is used in a powder form. In Japanese Patent Laid-Open Nos. 1-305042, 5-177140 and 5-11718.5, it is stated that a catalyst may be activated by gas phase reduction or by liquid phase reduction in a solvent exemplified by hydrocarbons such as paraffin, ethers such as dioxane, alcohols and esters. Gas phase reduction, however, requires an additional apparatus other than the reactor for reductive activation of the powdery catalyst and further a surface stabilizing treatment for preventing the resulting copper from being oxidized by air. Because of these drawbacks of gas phase reduction, liquid phase reduction is generally employed in the fluidized bed reaction system. In this case, it is generally agreed that reduction is carried out preferably at a temperature of from 150.degree. to 350.degree. C. until hydrogen absorption has stopped. Since heat removal is easy in the case of powdery catalysts, local overheating can easily be prevented.
On the other hand, when a fixed bed reaction system is employed, gas phase reduction is exclusively used for the reductive activation of a formed catalyst, and for industrial purposes, it is common practice to carefully reduce a catalyst at a given temperature while supplying an inert gas containing several to several dozens percents of hydrogen, to prevent local overheating due to rapid reduction.
Reduction of copper oxide with hydrogen is generally known to generate a heat of reduction of 20 Kcal per mole of copper oxide and reduced copper thus obtained has a very low thermal stability. For this reason, it is important to gradually reduce the copper oxide while controlling heat generation to prevent the deterioration of its catalyst performance. When using a formed catalyst, in particular, this is critical because heat removal is difficult.
It is, therefore, very likely that when the catalyst is activated by gas phase reduction with a high concentration of hydrogen in a short time, a rapid heat generation considerably degrades catalyst performance, and that when a large amount of catalyst is activated by reduction in a short time on an industrial scale, a rapid rise in temperature causes a very dangerous situation. For this reason, it is common practice to use a low concentration of hydrogen over a long period of time for activation of catalysts containing copper oxide by gas phase reduction. For example, Japanese Patent Laid-Open No. 61-161146 states that it takes as long as 4 to 14 days for catalytic activation by such reduction, suggesting a disadvantage of gas phase reduction in view of alcohol productivity.
Also, DT 1768313 discloses a method for reductive activation of a copper-zinc oxide catalyst, in which the catalyst is gradually reduced at a temperature of between 120.degree. and 240.degree. C. in a hydrogen-containing nitrogen gas stream and finally treated with high-pressure hydrogen at a temperature of between 250.degree. and 300.degree. C. for 1 to 2 hours. Japanese Patent Laid-Open No. 62-298457 states that a copper-chromium oxide catalyst can be activated by raising a temperature from 130.degree. C. to 200.degree. C. at a rate of 10.degree. C./hr and keeping it at 200.degree. C. for 12 hours in a nitrogen gas stream containing 1% by volume hydrogen. Also, DE 3443277A1 discloses a method for reductive activation of a copper-zinc oxide catalyst, in which the catalyst is reduced at 200.degree. C. in a nitrogen gas stream containing 5% by volume hydrogen for 16 hours and then further reduced with pure hydrogen at 200.degree. C. for 16 hours. Japanese Patent Laid-Open No. 61,178037 states that a copper oxide-magnesium silicate catalyst can be activated by reducing the catalyst at 200.degree. C. in a nitrogen gas stream containing 1 to 2% by volume hydrogen For 60 hours. In addition, Japanese Patent Laid-Open No. 1-127042, which discloses a method for reductive activation of copper-chromium oxide and reviews the prior arts, indicates that all methods require catalyst reduction temperatures of not lower than 150.degree. C.
Although gas phase reduction is commonly used in the fixed bed reaction system, several methods are also known to activate a catalyst precursor containing copper oxide by liquid phase reduction. For example, Japanese Patent Laid-Open Nos. 5-177140 and 5-117185 propose to activate a copper-zinc oxide catalyst at 200.degree. C. in an autoclave by the batch reaction method in liquid phase. Also, British Patent Publication No. 385625 describes a method of liquid phase reduction of a copper-chromium catalyst at 325.degree. C. in an ester flow of a liquid hourly space velocity of 8.0 by the fixed bed reaction system, followed by hydrogenating an ester. Also, Japanese Patent Laid-Open No. 47-14113 discloses a method of liquid phase reduction of a copper-chromium catalyst at 200.degree. C. in a lactone flow of a liquid hourly space velocity of 0.67 by the fixed bed reaction system, followed by hydrogenating a lactone. According to Japanese Patent Laid-Open No. 2-26611, the reduction of a catalyst containing copper oxide can be carried out after an ester, the starting material, has been supplied.
However, all these methods for catalytic activation by liquid phase reduction have proved to lack any advantages or superiority over those by gas phase reduction, as Japanese Patent Laid-Open No. 2-26611 describes that "reduction of the catalyst's copper component by these methods is not complete and somewhat difficult to control."